Jeremy Corbyn has promised to bring forward "very coherent", "credible" and "solid" policies that will help revive Labour's fortunes following its humiliating Copeland by-election defeat.

The Labour leader appeared to reject suggestions his leadership style was to blame for the party's poor performance, telling The Guardian "people obviously have a view, perceptions, about party leaders, and I have mine".

He suggested the defeat to the Tories in the Cumbrian seat of Copeland, which had been held by Labour since 1935, was down to demographic change, with the party's majority gradually tumbling in recent elections.

"There's also a longer term issue in Copeland that the Labour vote has actually been unfortunately going down for quite a long time and the area has changed; the area also needs an investment plan so it doesn't need to rely solely on nuclear but relies on other industries as well," he said.

He said his "democratic" approach to policy-making meant the process was "longer and slightly more cumbersome than calling in a few experts into my office to tell me what the policies should be."

Mr Corbyn went on: "We are going to be producing some very coherent policies on health, social care, mental health issues and funding. But crucially it's the economic questions.

"This is exciting involvement in an economic strategy-building across the whole country. It doesn't necessarily have the same media headlines immediately, but I tell you, in the longer term this is actually a more credible and more solid way of doing things."

Mr Corbyn earlier made a plea for unity at the Scottish Labour conference in Perth, stating: "The policies and ideas we are setting out are policies whose time has come.

"But to win that fight we need to remain united.

"United in our belief in our movement. United in our commitment to once again make our society fairer, better and more just.

"And united so we can provide our people with the hope they desperately need."

Mr Watson said those who backed Mr Corbyn in last year's leadership election should support him in good times and bad.

His comments came as shadow attorney general Baroness Chakrabarti blamed factors such as the media and division in the party for two poor performances in by-elections this week.

However, Labour MP Lisa Nandy, touted as a possible party leader, said it was "a severely inadequate response" to lay the blame beyond the party for its current problems.

Speaking on ITV's Peston On Sunday, Mr Watson said: "If I've got some frustrations, it's that those people that are Jeremy's cheerleaders, that made sure that he was elected a second time last September, they should be sticking with their leader in the bad times, not just the good.

"Dave Prentis has spoken out, but I'd say to you this morning, where's Len McCluskey defending his leader in this difficult time?

"It shouldn't be just down to me."

Mr Watson said it was not "a suicide approach" to stick with Mr Corbyn, as he urged the party to unite and ruled out a fresh leadership contest.

He added: "We can win an election with Jeremy Corbyn, but things have to change."

Gerard Coyne, who is challenging Mr McCluskey for leadership of the Unite union, also criticised his rival's failure to speak out.

"The silence of Len McCluskey is truly damning," Mr Coyne said. "He has driven Labour to the edge of the cliff and then disappeared in a puff of smoke as it tumbles over the edge."

But a Unite spokesman said: "Tom Watson is deputy leader of the Labour Party. It is his job to address the issues facing the party in the wake of the by-elections.

"Len McCluskey's job is to address the issues that are the foremost priorities for Unite's members."

Mr Corbyn appeared to back Mr McCluskey, telling Sky News: "Len McCluskey is going through a general secretary election at the present time and presumably is engaged in doing that.

"I had a very good conversation with Len McCluskey 10 days ago, a very good chat with him about lots of things, about economic policy, about industrial policy, about future employment law, and of course our attitude towards Brexit, and the crucial need to maintain tariff-free market access to Europe after we leave the European Union."

Asked repeatedly if he would be leader at the 2020 general election, in his last reply he said: "I've given you a very, very clear answer, yes."